Never use px.

leather/axis.py

@@ -71,7 +71,7 @@ def to_svg(self, width, height, scale, orient):
                 y2=six.text_type(y2),
                 stroke=tick_color
             )
-            tick.set('stroke-width', theme.tick_width)
+            tick.set('stroke-width', six.text_type(theme.tick_width))
 
             if orient == 'left':
                 x = label_x

leather/chart.py

@@ -175,7 +175,7 @@ def to_svg_group(self, width, height, margin=None):
                 fill=theme.title_color
             )
             label.set('font-family', theme.title_font_family)
-            label.set('font-size', theme.title_font_size)
+            label.set('font-size', six.text_type(theme.title_font_size))
             label.text = six.text_type(self._title)
 
             header_group.append(label)
@@ -221,9 +221,9 @@ def to_svg(self, path, width=600, height=600, margin=None):
         """
         Render this chart to an SVG document.
 
-        Technically, :code:`width` and :code:`height` are specified in SVG
-        "unitless" units, however, because font sizes resolve to pixels, it is
-        usually best to think of these as pixels.
+        Note: :code:`width` and :code:`height` are specified in SVG's
+        "unitless" units, however, it is usually convenient to specify them
+        as though they were pixels.
 
         :param path:
             Filepath or file-like object to write to.

leather/theme.py

@@ -7,20 +7,20 @@
 # Chart title
 title_color = '#333'
 title_font_family = 'Monaco'
-title_font_size = '16px'
+title_font_size = 16
 title_font_char_height = 20
 title_font_char_width = 9
 
 # Tick lines
-tick_width = '1px'
+tick_width = 1
 tick_size = 4
 tick_color = '#eee'
 label_color = '#9c9c9c'
 zero_color = '#a8a8a8'
 
 # Tick labels
 tick_font_family = 'Monaco'
-tick_font_size = '14px'
+tick_font_size = 14
 tick_font_char_height = 14
 tick_font_char_width = 8